1. Field of the Invention
The invention relates to a process for preparing cationic polyvinyl acetals, to the products obtainable thereby, and to their use.
2. Background Art
The preparation of polyvinyl acetals from the corresponding polyvinyl alcohols by means of polymer-analogous reaction with the corresponding aldehydes has been known since as early as 1924, the ensuing period having seen the use of a multiplicity of aldehydes to prepare the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a 3-stage operation (polyvinyl acetate→polyvinyl alcohol→polyvinyl acetal), giving products which along with vinyl acetal groups, also contain vinyl alcohol units and vinyl acetate units. Commercial significance has been acquired in particular by polyvinyl formal, polyvinyl acetal, and polyvinyl butyral.
There are numerous applications in which the adhesion of polyvinyl acetal films to a substrate is inadequate. To improve the adhesion of polyvinyl acetal coating compositions, for example, polyethyleneimine is added. Polyethyleneimine has the drawback, however, of poor compatibility with nitrocellulose, a commonly used solvent for coating materials.
Cationic polyvinyl acetals are employed in a multiplicity of applications. JP-A 57-077051 describes using cationic polyvinyl butyral as an interlayer for safety glass. The cationic groups are introduced by copolymerizing trimethyl(3-acrylamido-3,3-dimethylpropyl)ammonium chloride with vinyl acetate, prior to hydrolysis and acetalization. JP-A 06-279539 describes using cationic, acetalized polyvinyl alcohol as a protective colloid in emulsion polymerization. The cationic group is introduced by means of polymer-analogous reaction of the acetal groups with a cationic compound. In JP-A 2000-225768 the cationic group is also introduced by means of polymer-analogous reaction of the acetal group. That product is used to coat inkjet papers.
These prior-art processes have several distinct disadvantages, including attachment of the cationic group via hydrolysis-sensitive ether, ester or amido groups, and the need for an additional process step in the polymer-analogous reaction. ACTA POLYMERICA 43, 230-34 (1992) describes a process for preparing cationic polyvinyl acetal, wherein in a first step, vinyl acetate is copolymerized with dimethyldiallylammonium chloride in anhydrous methanol at a solids content of 3 to 4 mol %. The copolymer obtained by this step is subsequently hydrolyzed and acetalized. The very low conversion in the copolymerization makes it necessary to remove the residual monomer by repeated reprecipitation.